1. Field of the Invention
The present invention relates to a method for producing xylitol from lignocellulosic hydrolysates without detoxification, in which the xylose fermentation strain Candida sp. shows high furfural tolerance and is used to convert xylose into xylitol from various source of non-detoxified lignocellulosic hydrolysates. The overall utilization of xylose in hydrolysate reach over 95%.
2. Related Art
Xylitol is a rare sugar that exists in low amount and is the constituent of many vegetables and fruits. Xylitol is also one of the intermediate metabolites in the sugars metabolism of mammalian, and its chemical property belongs to the pentitols. The reason why xylitol attracts global attention is that it is a natural sweetener having equivalent sweetness to sucrose with the calorie of merely 60% of sucrose. Medical studies have shown that, xylitol is helpful for the prevention of dental caries, because it is not easily utilized by Streptococcus mutans and other microorganisms that may cause tooth decay, and also has the function of maintaining acid-base balance in the mouth. Furthermore, studies also point out that xylitol is rapidly metabolized to generate energy, but the metabolism in human body dose not need insulin participation, so it is widely used as substitute of sucrose in the nutrition for diabetics in clinic presently. Generally, the development of industrial production of xylitol has applications for at least three major industries, that is, food processing industry (for example, baking industry, emulsifiers, stabilizing agents, and chewing gum), odontological prevention and control (dental caries prevention, promotion of tooth re-mineralization and rehardening), and pharmaceutical industry (prevention of upper respiratory tract infection, as sweetener for its high-sweetness and low-calorie properties, nutraceuticals, and vitamin formulations).
Presently, the method of industrial mass production of xylitol includes the following steps. The lignocellulosic biomass material enriched with hemicellulose is pretreated by acid hydrolysis and converted into a hydrolysate with xylose as the main component. Next, the xylose-rich hydrolysate is hydrogenated at high temperature and high pressure with the catalysis of nickel metal and then to produce xylitol from the conversion of xylose. The yield of xylitol produced by this chemical synthesis is about 40-50%, and at the same time, all the sugars present in the hydrolysate are also reduced to their corresponding sugar alcohols. Therefore, in addition to xylitol, other sugar alcohols, such as arabitol and sorbitol, may also exist in the product. These sugar alcohols have similar chemical properties and are relatively difficult to be separated. Furthermore, the production process of chemical synthesis is complex and consumes a great amount of energy, and the equipment cost is high, so the price of xylitol cannot be decreased. In order to decrease the production cost and meet the increasing market demand for xylitol, the industry is actively developing a high yield but low energy-consumption alternative for xylitol production.
The bioconversion method for xylitol production by fermentation of lignocellulosic hydrolysate using microorganisms is the most advantageous and competitive alternative presently, in which by using the naturally occurring xylose-fermenting microorganisms, the xylose is directly converted into xylitol through the physiological metabolism of the microorganisms, and then the xylitol product is then recovered by purification and crystallization. In addition to high production yield, bioconversion method also has the advantage in the elimination of the risk that the xylitol product may be contaminated by heavy metal by the chemical synthesis method. As for the regulation standard for food additives, the xylitol product produced by bioconversion method is relatively safe.
In the relevant literatures currently collected, the type of the hydrolysates discussed in the studies of converting xylose into xylitol by microorganisms include corncob, corn fiber, sugarcane bagasse, hardwood, eucalyptus, walnut shell, brewer's spent grain, prairie grass, wheat straw, and rice straw, etc., among which, there is a large difference in relevant xylitol yield (0.2-0.8 g/g).
Generally, the lignocellulosic biomass material mainly contains 60-80% of cellulose, hemicellulose, and 15-25% of lignin, in which hemicellulose is required to be converted into pentoses (mainly xylose) through a pretreatment process, and then further converted into xylitol by microorganism fermentation. Presently, the pretreatment technology for converting hemicellulose into saccharides mostly adopts high-temperature and high-pressure thermal chemical pretreatment technologies, such as, dilute acid hydrolysis, dilute acid-catalyzed steam explosion to decompose hemicellulose into xylose. During the reaction of such pretreatment technology, generally, a certain proportion of raw material and an aqueous solution are firstly filled into a reactor, and then 1-3% (w/w) of dilute sulfuric acid is added under high-temperature and high-pressure reaction conditions, and the liquid obtained after the reaction is so-called as hydrolysate. In addition to the release of sugars, the pretreatment also generates certain amounts of fermentation inhibitors, such as, acetic acid, furfural, and hydroxymethyl furfural, accompanied with different reaction conditions. Therefore, presently, the xylose-rich hydrolysate obtained by pretreatment is usually treated with detoxification technology, such as, overliming, active carbon adsorption, and ion exchange resin, alone or in combination, such that hydrolysate is subjected to fermentation with microorganisms successfully and converted xylose into the xylitol. For example, for the mostly used overliming method (as shown in FIGS. 1A and 1B), the conditioning of the xylose-rich hydrolysate includes heating, adding excessive lime, solid-liquid separation, and adding an acid agent to adjust the pH value to be weakly acidic, etc. The generated gypsum sludge is required to be further treated and disposed. Because during the conditioning of the overliming method, xylose loss is generally caused, and calcium sulfate sludge is generated, additional cost and equipments for treatment and disposal are required, thus increasing the production cost of xylitol. By comparison, the process without detoxification is simple, and it is only required to add an alkali agent to adjust the pH value of the xylose hydrolysate to be weakly acidic. However, the non-detoxified hydrolysate may contain high concentration of fermentation inhibitors, such as furfural and sulfate ion, so that the difficulty of converting xylose into xylitol by fermentation is increased relatively. Therefore, it is an important issue for reduce the xylose loss as well as the production cost for xylitol to improve the competitiveness of bioconversion-based xylitol production.